It is well known that the fracture mechanism of micron superabrasive powders, defined as diamond and cubic boron nitride (CBN) powders, plays a determinant role in the abrasive process. The fracture mechanism of superabrasive particles is controlled by their crystalline structure (i.e. monocrystalline vs. polycrystalline) and by the nature and concentration of crystal growth defects (i.e. pre-existing fractures, voids, mechanical stresses and impurities). Consistent abrasive performance is achieved only by employing micron superabrasive powders whose properties are precisely defined and controlled. Therefore, it is crucial that micron superabrasive powder products be designed and manufactured to possess a specific set of chemical and physical properties that are responsible for their performance in a particular application. It is equally important that the end user understand the chemical and physical properties associated with different micron superabrasive powder types/products, and make an informed decision to select the product that performs best for their application. Whether used as abrasive or non-abrasive (i.e. feedstock for PCD manufacturing), mechanical strength and fracture characteristics are primarily responsible for the performance of a micron superabrasive powder type/product.
Generally, sub-sieve powders (powders that are smaller than 400 mesh) are considered micron powders. However, in the size range from 40 microns (approx. 400 mesh) to 80 microns (approx. 200 mesh), fine mesh sizes overlap with coarse micron-sizes. It is well known that the fracture mechanism of micron superabrasive particles plays a determinant role in any abrasive process. During abrasive action, the edges and points of micron superabrasive particles tend to dull. Progressive dulling of the particles leads to increased mechanical and thermal stresses at the abrasive-workpiece interface. If allowed to continue, this process leads to catastrophic failure of the micron superabrasive particles and workpiece damage. To avoid catastrophic failure, the micron superabrasive particles must be able to microfracture under severe mechanical stresses and develop new fresh cutting edges and points in a so-called “self sharpening” mechanism.
The fracture mechanism of micron superabrasive particles is controlled by crystalline structure (i.e. monocrystalline vs. polycrystalline) and by the nature and concentration of crystal growth defects (i.e. pre-existing fractures, voids, mechanical stresses and impurities). Virtually all synthetic mesh micron superabrasive powders are commercially produced via the catalytic high pressure-high temperature (HPHT) synthesis process. Manufacturing of micron superabrasive powders by the same process has been proven difficult, impractical and expensive. Micron superabrasive powders represent a by-product of the diamond or CBN synthesis process and are produced by milling of mesh size powders. Consequently, some of the characteristics of the starting mesh powders (so-called diamond or CBN feeds), are mirrored in the resulting micron superabrasive powders.
The intrinsic properties of the diamond or CBN crystal are determined by the nature (static vs. dynamic) and particularities of the HPHT synthesis process. For a given graphite-catalyst system, the kinetics of the catalytic HPHT synthesis process (i.e., nucleation and growth rates) is controlled by thermodynamic parameters—pressure and temperature. Furthermore, the nucleation and growth rates control the nature and amount of crystal growth defects (pre-existing fractures, voids, mechanical stresses, impurities) which, in turn, are responsible for the mechanical strength of the crystal. Therefore, there is a direct relationship between the crystal characteristics (size, shape, mechanical strength) and the nucleation and growth rates, as follows:                Low nucleation and growth rates produce larger, well developed (regular shaped) crystals with a low level of crystal growth defects and high mechanical strength.        High nucleation and growth rates produce smaller, poorly developed (irregular shaped) crystals with a high level of crystal growth defects and low mechanical strength.As a general rule, the level of crystal growth defects is strongly related to the synthesis process, while the distribution of crystal strength within the population is related to the post-synthesis processing of mesh powders (shape sorting, magnetic separation, etc). A brief summary of the particularities of some of the most frequently practiced catalytic HPHT diamond synthesis processes is presented in Table 1.        
TABLE 1Cubic/ApposedBelt, PressHexagonalAnvilsHPHT DeviceStraightCurvedPressPressMetal CatalystNi—FeCo—FeNi—FeNi—Co—MnNi—MnGraphite-Metal chargeDiscs/PowderDiscs/PowderPowderHPHT cycleLong-ModerateModerate-ShortShort-V. ShortGrowth rateLow-ModerateModerate-HighHigh-V. HighCrystal growth defectsLow-ModerateModerate-HighHigh-V. High
The characterization of micron superabrasive powders is a difficult and complex task, involving the evaluation of the properties of a very large number of particles. There are no standard techniques in the prior art for measuring the mechanical strength and/or fracture characteristics of micron superabrasive powders, by either static or dynamic methods. Instead, the mechanical strength and fracture characteristics of micron superabrasive powders are controlled indirectly, by controlling the feed type and quality (i.e., metal bond diamond/CBN using a belt press or cubic press synthesis process; or resin bond diamond/CBN using a belt press or cubic press or opposed anvils press synthesis process). Furthermore, the concentration of residual crystal growth defects, as well as the particle shape and surface texture of the resulting micron superabrasive powders, can be significantly modified through a number of mechanical, chemical and thermal processes that are incorporated into the micronizing process.
There exists a need for a methodology and apparatus for characterization of different micron superabrasive powder types/products with respect to fracture strength and fracture characteristics.